1. Field of the Invention
The features of the present invention relate to position determination systems and more particularly to a method and apparatus for determining a position of a mobile device or mobile station using global positioning system (GPS) data and expected user altitude on data.
2. Background Art
To determine a position of a mobile device a Global Positioning System (GPS) uses measurements from several satellites to compute position in three (3) dimensions (i.e., latitude, longitude, and altitude). For the ideal situation, four (4) or more satellites are required for these measurements. However, in certain situations, there may not be enough satellites to accurately determine an altitude. For example, in some cases only three (3) GPS satellites are available. In these cases, a measurement is sometimes available in the form of an altitude estimate, and in this case four (4) measurements (3 GPS+1 altitude) can be used. This can include blocked GPS signal environments (e.g. indoors, urban canyon), or even moderate GPS environments, where at some point during startup, only three (3) GPS measurements are ready. Even in some cases when four (4) GPS satellites are available, the geometry of those satellites is poor, or the solution is otherwise not high enough quality to be usable, and the use of altitude aiding can provide an additional measurement to improve the accuracy and reliability of that solution. For this situation, many commercial GPS receivers offer a fixed altitude mode for situations where the altitude of the receiver is available from external means, such as knowledge that the receiver is being used on the surface of the ocean in a maritime GPS receiver. Also, mobile stations sometimes remember a previously calculated altitude, and use that for altitude aiding; however, obvious problems with this technique include the inability to determine an altitude for a first positioning attempt or when the mobile station has moved significantly since the previous altitude was calculated.
The fixed altitude technique, while useful in some situations, cannot address the general problem of determining position over uneven and/or unknown terrain. The use of terrain elevation maps is disclosed in U.S. Pat. No. 6,429,814 B1 and U.S. Pat. No. 6,590,530 B2. However, these devices are impractical and require substantial resources due to the volume of terrain elevation data for the entire earth. These worldwide altitude databases are often stored on a server and, due to their size, are rarely in a mobile station.
Therefore a need exists for a method and apparatus to accurately provide elevation data that is more practical and that requires less memory than the presently available systems.